TECTONIC FLOW MÉLANGE AND OLISTOSTROMES IN THE FRANCISCAN COMPLEX: WHICH IS MORE ABUNDANT?

Recent papers have challenged the concept that shale-matrix mélange in the Franciscan Complex of California is the product of backflow up the plate interface zone, the “subduction channel.” Instead, these workers argue the mixed rocks, which form a large part of the Central Belt north of San Francisco, are primarily the result of surficial slumping, sliding, and mass flows collectively known as olistostromes. That olistostromal processes operate on trench walls is not a debate, but their volumetric abundance is, for little chaotic material has been recovered by deep sea drilling of trench walls. Moreover, blueschist block-bearing material is found at locations in central California south of the famed exposures at San Simeon to north of the Mendocino triple junction (MTJ) where subduction is ongoing. With restoration of the San Andreas Fault, the belt of Franciscan mélange appears to have been continuous for more than 1500 km. South of the MTJ, the belt is up to several tens of km wide. The belt is best described as dominated by mélange for it also contains bedded materials that were deposited on top as trench slope cover (mostly siltstone and shale) and local slope basins and submarine channels that are sand-rich. These sequences, at most zeolite facies, contain small volumes (100± m scale) of olistostromal deposits. Explaining the blueschist-block bearing mélange as solely a product of olistostromal mixing also posits a phantom, but extensive source area for the blueschists and rare eclogites that make this mélange so famous. Metamorphism of the shale matrix (illite and chlorite crystallinity, pumpellyite± lawsonite) and included graywacke blocks indicates much of the mélange returned from depths of 10+ km. The cataclastic deformation and metasomatic alteration of the margins of mafic blocks that are frozen in various stages of boudinage must have occurred in an environment with high fluid pressures as the enveloping shale matrix was dynamically metamorphosing at temperatures of ~150°C. Only subduction shearing and backflow up the subduction channel can explain the incorporation, deformation, and retrograde metamorphism of the blueschist blocks. Upflow reached the surface and some material was incorporated into surficial olistostromes, but this was volumetrically minor in the Franciscan Complex.